Chemical sensors can be used to detect ultra-trace explosive analytes. The critical importance of detecting explosives in a wide variety of areas, such as minefields, military bases, remediation sites, and urban transportation areas, has spurred significant research into developing and improving chemical sensors. Safety screenings also encompass counter-terrorism efforts, such as personnel or baggage screening, facility protection and cargo screening. In addition to such safety screenings, the detection of explosive analytes is important for forensic investigations, such as the examination of post-blast residue.
Typical chemical sensors are small synthetic molecules that produce a measurable signal upon interaction with a specific analyte. Chemical sensors are cost effective and can succeed where other techniques fail to detect explosives. Modern land mines, for example, are encased in plastic and can be missed by metal detectors. Trained dogs are effective, but are expensive and difficult to maintain. Other detection methods, such as gas chromatography coupled with a mass spectrometer, surface-enhanced Raman, nuclear quadrupole resonance, energy-dispersive X-ray diffraction, neutron activation analysis and electron capture detection are highly selective, but are expensive and not easily adapted to a small, low-power package.
Conventional chemical sensors also have limitations that render them ineffective under some conditions. Sensing TNT (2,4,6-trinitrotoluene) and picric acid in groundwater or seawater is important for the detection of buried, unexploded ordnance and for locating underwater mines, but most chemical sensor detection methods are only applicable to air samples because of interference problems that are encountered in complex aqueous media. Such conventional chemical sensors are therefore inefficient in environmental applications for characterizing soil and groundwater contaminated with toxic TNT at military bases and munitions production and distribution facilities. In addition, conventional chemical sensors, such as highly π-conjugated, porous organic polymers, can be used to detect vapors of electron deficient chemicals, but require many steps to synthesize and are not selective to explosives.
Many conventional chemical sensors are not amenable to manufacture as inexpensive, low-power portable devices. Typical chemical sensing methods are limited to vapor phase detection, which is disadvantageous given the low volatility of many explosives. Nitroaromatic explosives such as TNT have moderate vapor pressures (7×10−6 Torr at room temperature), but at low surface concentrations, the vapor concentration of TNT molecules is significantly lower than its equilibrium vapor pressure. Nitramine high explosives, such as RDX (cyclotrimethylenetrinitramine, also known as cyclonite, hexogen, and T4) and HMX (cyclotetramethylene-tetranitramine) have substantially lower vapor pressures (5×10−9 and 8×10−11 Ton, respectively) than TNT, which makes vapor detection of these compounds very difficult. Vapor concentrations can be lowered even more when these compounds are enclosed in a bomb or mine casing, or when they are present in a mixture with other explosives. The broad array of nitrogen-based explosives has rendered it difficult to provide a single method whereby multiple types of explosives may be detected.
While efficient explosives detection has always been a predominating concern, there exists a renewed urgency for development of rapid and highly sensitive detection of organic, nitrogen-based explosives, including nitroaromatic compounds, nitramine-based explosives and nitrate ester-based explosives. In addition to detecting TNT, for example, detection of the nitrogen-based plastic explosives compounds associated with improvised explosives devices (IEDs), namely RDX and PETN (pentaerythritol tetranitrate), has life-saving applications in a vast array of applications, such as forensic, military, and civilian homeland security purposes.
Practical blue LEDs have been developed after decades of effort. Early blue LEDs developed in the 1970s provided little output, and in the 1990s high brightness blue LEDs were demonstrated with Group III-V materials. As with other Group III-V devices, blue LEDs made from Group III-V materials are more expensive and more difficult to fabricate than silicon fabrications.